The invention relates to a method of feedback controlling the air-fuel ratio of mixture by means of an air-fuel ratio sensor positioned in the exhaust gases from an engine for automobiles or the like.
In a known type of feedback air-fuel ratio control method employing reducing and oxidizing catalysts, it is necessary to control the air-fuel ratio of mixture in such a manner that the center value of the controlled air-fuel ratio or the controlled center air-fuel ratio comes into a very narrow range of air-fuel ratios around the stoichiometric ratio required by the reducing and oxidizing catalysts as shown in FIG. 1.
However, the controlled center air-fuel ratio will be affected by the characteristics of an air-fuel ratio sensor and the exhaust gas composition characteristic is dependent to a considerable extent on the variations in characteristics caused by different air-fuel ratio sensors.
The air-fuel ratio sensor characteristics which affect the controlled center air-fuel ratio include the output characteristic (hereinafter referred to as a static characteristic) which is a stepwise relation between the sensor output and the air-fuel ratio as shown in FIG. 2 and another characteristic (hereinafter referred to as a dynamic characteristic) involving differences in response delay between the sensor output when the air-fuel ratio is changing from the rich side (no oxygen is present in the exhaust gases) of a desired (stoichiometric) air-fuel ratio to the lean side (oxygen is present in the exhaust gases) and the sensor output when the air-fuel ratio is changing in the reverse direction. These characteristics differ with different sensors or different use conditions and the resulting controlled center air-fuel ratio also differs similarly. As a result, the exhaust gas composition characteristic also differs with different sensors or different use conditions.
In accordance with the present invention, it has been found that the output of the air-fuel ratio sensor changes abruptly in response to a threshold or a comparison voltage (corresponding to a predetermined air-fuel ratio) of a comparator circuit for determining whether the air-fuel ratio is great (lean) or small (rich) as compared with the predetermined air-fuel ratio, that if the air-fuel ratio sensor is warmed sufficiently, the sensor output characteristic (static characteristic) will not vary greatly with different sensors or different use conditions and that the previously mentioned dynamic characteristic is a major cause of variations in the controlled center air-fuel ratio.